Crossed field electron discharge device having a non-uniform interaction space



May 21, 1968 A. REDDISH 3,384,782

CROSSED FIELD ELECTRON DISCHARGE DEVICE HAVING A NON-UNIFORM INTERACTION SPACE Filed Dec. 2, 1965 km aml HTTDRMFYS United States Patent 0 3,384,782 CROSSED FIELD ELECTRON DISCHARGE DEVICE HAVING A NUN-UNKFORM IN- TERACTION SPACE Alan Reddish, Pinner, England, assignor to The M-O Valve Company Limited, London, England, a British company Filed Dec. 2, 1965, Ser. No. 511,036 Claims priority, application Great Britain, Dec. 3, 1964, 49,296/64 7 Claims. (Cl. 31539.3)

This invention relates to electron tubes of the kind in which in operation electrons flow in a space in which there is established a system of crossed magnetic and electrostatic fields.

Various forms of electron tube of this kind have been proposed for use as amplifiers. A difficulty commonly encountered in such cases is that noise signals constituted by fluctuations in the electron flow are considerably amplified by processes inherent in the type of electron flow utilised, and in consequence satisfactory operation can normally only be achieved when using input signals of relatively high level.

It is an object of the present invention to provide electron tubes of the kind specified which are improved in this respect.

According to the invention, an electron tube has an evacuated envelope within which is disposed a space for electron flow which is of uniform cross-section in a set of planes perpendicular to a given plane, said space being defined by an electrode system which includes a thermionic cathode having an electron emissive surface and a further electrode having an operating surface which faces the emissive surface so that said surfaces effectively constitute lateral boundaries of said space, the tube being arranged to operate with the further electrode held positive with respect to the cathode so as to establish an electrostatic field within said space and with a magnetic field established in said space perpendicular to all the planes of said set, whereby in operation electrons emitted from the emissive surface are formed into a beam which travels generally along said space, the tube also having means for applying an input signal so as to modulate the electron beam in said space and means for deriving an output signal by demodulation of the electron beam, and the operating surface of the further electrode being spaced from the emissive surface non-uniformly along the length of the emissive surface in accordance with the conditions specified below.

Consider the geometry of the space between the cathode and the further electrode, in terms of the uniform crosssection referred to above. The traces in this cross-section of said lateral boundaries will be a pair of lines effectively constituted by the traces of the emissive surface and the operating surface of the further electrode; the qualification effectively is included because either of these surfaces may have formed in it gaps of a nature such that their presence does not materially disturb the form of the equipotentials established in operation in the space between the surfaces. The traces of the lateral boundaries corresponding respectively to the cathode and the further electrode will be denoted C and F respectively. It will be appreciated that over at least part of C the normals to C will intersect F, and we define as aoreference point the forward end of said part of C (that is the end which corresponds to that end of said space towards which the electron beam flows in operation). We define the position of any point A on C by its distance X, measured along C, from the reference point, positive values of X corresponding to displacement along C in a direction opposite to that of the electron flow and the value of X "ice at the rear end of C being denoted L. For any such point A we define the spacing 5 between C and F as the distance of F from the point A measured along the normal to C at the point A, the value of S when X is zero being denoted D. We then specify the conditions that L is greater than D and that the variation of S should be at least approximately in accordance with a function of X such that, over the range of values of X from zero to 0.9L, S is finite, the first differential coefiicient dS/dX is never negative and is never greater than 1.8, the second differential coefficient d S/alX is never negative and the value of S is not greater than 2D when X is equal to 0.6L and is not less than l.5D when X is equal to 0.9L.

The use of a non-uniform spacing as specified above ensures that when the cathode is operated so that the electron emission is space charge limited, as is normally desirable, the electron emission density is relatively uniform along the length of the electron emissive surface. As a result, the electron beam is inherently less noisy than would be the case if a uniform spacing between the emissive surface and the operating surface of the further electrode were used. By reason of this, and by virtue of arranging for the electron beam to be modulated by the input signal in the space in which the beam is formed, it is possible to obtain satisfactory operation when using input signals of relatively low level.

One electron tube in accordance with the invention and suitable for use as a pulsed amplifier in the frequency band 8 to 11 gc./s. will now be described, by way of example, with reference to the accompanying drawing which is a sectional elevation of the tube.

The tube includes an evacuated envelope 1 in which is mounted an indirectly heated thermionic cathode 2, a sole 3, a further electrode 4, and a collector electrode 5, the electrodes 2, 3 and 4 together defining a space for electron flow in which electrons emitted from the cathode 2 are formed into a beam which travels towards the collector electrode 5.

The cathode 2 essentially comprises an elongated hol- 'low box 6, of approximately square cross-section, along the length of the interior of which extends a spiral resistance heater 7. The emissive portion of the cathode 2 is in the form of a porous plate 8 of sintered tungsten which constitutes one side of the box 6, thus providing a rectangular planar emissive surface of length 3.937 inches and breadth 0.309 inch. The other sides of the box 6 are made of molybdenum and the box 6 is supported on a molybdenum web 9 which projects from the centre of the side of the box 6 remote from the plate 8 and extends lengthways along the cathode 2. The heater 7 is supported on metal plates 10 which extend lengthways along the cathode 2 and are disposed one on either side of the box 6. The web 9 and the plates 10 serve respectively as connectors to the cathode 2 and the heater 7 and are connected to leads 11 sealed insulatingly through the envelope 1.

Sideways escape of electrons from the space for electron flow is inhibited by means of two further metal plates 12 extending lengthways along the cathode, one on either side of the box 6, outside the heater support plates 10, the plates 12 being connected to a further lead 13 which is sealed through the envelope 1. A further beam shaping electrode 14 is provided at the upstream end of the space for electron flow in the form of a metal plate which is attached to the relevant end of the box 6 and lies in a plane at approximately 120 to the emissive surface of the cathode 2.

The components parts of the cathode 2 are insulatingly bolted together and the cathode assembly is mounted on ceramic support members 15 which in turn are carried by metal brackets 16 which are bolted to a copper block 17 which forms a part of the envelope 1.

The sole 3 is in the form of a rectangular metal plate of width .320 inch and length 1.35 inches which is carried by a support 18 of insulating material which is mounted on the copper block 17 so that one main face of the sole 3 is coplanar with the emissive surface of the cathode 2, the width of the sole 3 being parallel to and in register with the width of the emissive surface and the adjacent ends of the sole 3 and the cathode 2 being spaced .020 inch from one another. A lead 19 to the sole 3 is sealed insulatingly through the envelope 1.

The further electrode 4 includes a generally rectangular copper plate 20 which is secured to the copper block 17 so that the face of the plate 20 which is remote from the block 17 faces the emissive surface of the cathode 2 and the corresponding surface of the sole 3. The plate 20 extends substantially from the end of the cathode 2 remote from the sole 3, along the whole length of the cathode 2, and for a distance of one inch opposite the sole 3, and is disposed with its width parallel to, and in register with, the widths of the cathode emissive surface and the sole 3. The further electrode also incorporates a delay line 21 of the folded waveguide type which is .mounted in a rectangular slot which extends centrally along the length of the plate 20 from the end of the cathode 2 adjacent the sole 3 for a distance of 2.95 inches opposite the cathode 2, the delay line 21 being disposed with its open side fiush with the surface of the plate 20.

It will be appreciated that the emissive surface of the cathode 2, the corresponding surface of the sole 3, and the adjacent surface, that is the operating surface, of the further electrode 4, together constitute the lateral boundaries of the space for electron flow, the space being of uniform cross-section in a set of planes perpendicular to the shorter sides of the emissive surface. The profile of the operating surface of the further electrode 4 is such that, measured along the normals to the emissive surface of the cathode 2, the spacing between the emissive surface of the cathode 2 and the operating surface of the further electrode 4 in each such cross-section varies along the length of the cathode 2 as follows:

At the end of the cathode 2 adjacent the sole 3 the spacing is .079 inch, at a point 3.150 inches from the end of the cathode 2 adjacent the sole 3 the spacing is 0.127 inch, at a point 3.74 inches from the end of the cathode adjacent the sole 3 the spacing is 0.236 inch, at the end of the cathode 2 remote from the sole 3 the spacing is 0.435 inch, and between each adjacent pair of these points the spacing changes linearly. At the end of the further electrode 4 remote from the sole 3 the copper block 17 is shaped to provide a continuation of the profile of the adjacent part of the further electrode 4.

The spacing between the further electrode 4 and the sole 3 has a constant value of 0.079 inch.

It will be appreciated that in this embodiment, in terms of the parameters defined above, C is a straight line, L has a value of 3.937 inches, D has a value of 0.079 inch, S has a value of 1.45D when X is equal to 0.6L, S has a value of 2.5D when X is equal to 0.9L, and the maximum value of dS/dX for values of X from O to 0.9L is 0.185.

At its end farther from the sole 3 the delay line 21 is provided with an input connector in the form of a ridged waveguide 22, one end of the Waveguide 22 being disposed in an aperture in the copper block 17 so that the interior of the waveguide 22 communicates with the relevant end of the delay line 21. At its other end the delay line 21 is similarly provided with an output connector in the form of a ridged waveguide 23.

The collector electrode is in the form of a hollow copper block 24 through which cooling water is circulated in operation via inlet and outlet ports 25. The 'block is insulatingly mounted from the copper block 17 and is positioned adjacent the end of the space between the sole 3 and the further electrode 4 remote from the cathode 2, with the part of the sole 3 which projects 4 beyond the further electrode 4 disposed in a recess 26 cut into the block 24.

To further cool the tube in operation, the block 17 is provided on each side with a longitudinally extending channel 27 each of which is closed by a cover plate 28, water being passed through the channels 27 via a port 29. To assist cooling, a series of twisted metal lugs 30 is provided along the length of each channel 27.

In operation, the tube is disposed between the pole pieces of a permanent magnet (not shown) so that the space for electron flow lies in a uniform magnetic field H of strength 1700 oersteds directed parallel to the shorter sides of the emissive surface of the cathode 2, the sense of the magnetic field H being such that an electron in the space defined by the cathode emissive surface and the further electrode 4 travelling away from the cathode 2 in a direction perpendicular to the emissive surface will experience, due to the magnetic field, a force directed parallel to the longer sides of the emissive surface towards the narrow end of the space between the emissive surface and the further electrode 4.

Taking the potential of the further electrode 4 to be zero, the other electrodes are operated at pulsed potentials as follows: the cathode 2, 7.77 kilovolts negative, the sole 3, 7.77 to 10 kilovolts negative; and the collector electrode 5 at zero volts, A voltage is applied to the heater 7 such that the cathode 2 is heated to a sufiiciently high temperature to,v ensure that the electron emission is space charge limited. The electrons emitted from the cathode 2 are formed, under the influence of the electrostatic and magnetic fields established in the space between the emissive surface of the cathode 2 and the further electrode 4, into a beam which flows along the space towards the narrower end of the space, through the space between the sole 3 and the further electrode 4, and into collision with the collector electrode 5.

In addition, an input signal is applied to the input connector 22 thereby causing an electromagnetic wave to travel along the delay line 21. The operating conditions are such that the phase velocity of the wave propagated along the delay line 21 is appropriately matched to the phase velocity of the wave which grows on the electron beam in accordance with the phenomenon explained by slipping stream theory for Brillouin beams; as a result, the amplitude of the wave in the system builds up as it travels along the space between the delay line 21 and the emissive surface of the cathode 2 so that an amplified replica of the input signal may be extracted via the output connector 22.

In order to reduce the risk of oscillation occurring in operation, the device described above by way of example may be modified either by the provision of an attenuating region along the delay line 21, or by braking the delay line 21 at an intermediate point along its length, thus dividing the delay line 21 into separate input and output sections, and providing the adjacent ends of the input and output sections of the delay line 21 with reflectionless terrninations.

It will be appreciated that with the arrangements described above, by way of example, the gain obtainable is governed by the electrical length of the delay line 21. In alternative arrangements in accordance with the invention, if the length of the emissive surface of the cathode is not long enough to accommodate a suificient length of delay line to achieve a desired gain, the modulated electron beam appearing at the narrower end of the space between the further electrode and the emissive surface of the cathode may be directed longitudinally along the length of a further elongated interaction space wherein amplification is obtained in the manner employed in known forms of crossed field travelling wave tube. In such an arrangement the output is derived from that end of the delay line associated with the further interaction space which is further from the cathode, and the adjacent ends of the delay line associated with the cathode and the delay line associated with the further interaction space are suitably each provided with a refiectionless termination.

I claim:

1. An electron tube including: a thermionic cathode having an electron emissive surface; a further electrode having an operating surface which faces said emissive surface so that the cathode and the further electrode define a space for electron flow whose lateral boundaries are effectively constituted by said emissive surface and said operating surface, said space being of uniform cross-section in a set of planes perpendicular to a given plane and said operating surface being non-uniformly from said emissive surface such that in each cross-section the parameters L, D, S and X defined below satisfy the following conditions: L is greater than D; and over the range of values of X from zero to 0.9L, S is finite and increases with X such that the first difiYerential coefii-cient dS/dX is never negative and is never greater than 1.8, the second differential coefiicient ai S/dX is never negative, and the value of S is not greater than 2D when X is equal to 0.6L and is not less than 1.5D when X is equal to 0.9L where, in that cross-section, L is the length of that part of the trace C of the emissive surface over which the normals t0 the trace C intersect the trace F of the operating surface; S is the distance between the trace C and the trace F at any point A on said part of C measured along the normal to C at that point A; X is the distance of any such point A measured along C from one of said part of C and D is the value of S when X is zero; means for establishing a magnetic field within said space perpendicular to all the planes of said set so that, in operation, with the further electrode held at a positive potential with respect to the cathode so as to establish an electrostatic field in said space, electrons emitted from the emissive surface are formed into a beam which travels generally along said space towards its narrower end; means for applying an input signal to modulate the electron beam within said space, and means for deriving an output signal by demodulation of the electron beam.

2. An electron tube according to claim 1 wherein said means for applying an input signal comprises a delay line extending generally in the direction of the electron flow in said space over at least a major part of the length of said space, the input signal being applied to the end of said delay line further from the end of said space towards which the electron beam flows in operation.

3. An electron tube according to claim 2 wherein the delay line forms part of said further electrode.

4. An electron tube according to claim 3 wherein at least over the length of the delay line the traces of C and F are rectilinear.

5. An electron tube according to claim 4 wherein the value of S at the end of the delay line further from the end of said space towards which the electron beam flows in operation is at least approximately 1.5 times the value of S at the other end of the delay line, and the value of S at said other end of the delay line is :at least approximately one fortieth of the distance between the normals from the emissive surface to the two ends of the delay line.

6. An electron tube according to claim 2 wherein said delay line is in the form of a folded waveguide.

7. An electron tube according to claim 2 wherein an output signal is derived from the end of the delay line nearer the end of said space towards which the electron beam flows in operation.

No references cited.

HERMAN KARL SAAL'BACH, Primary Examiner.

S. CHATMON, JR., Assistant Examiner. 

1. AN ELECTRON TUBE INCLUDING: A THERMIONIC CATHODE HAVING AN ELECTRON EMISSIVE SURFACE; A FURTHER ELECTRODE HAVING AN OPERATING SURFACE WHICH FACES SAID EMISSIVE SURFACE SO THAT THE CATHODE AND THE FURTHER ELECTRODE DEFINE A SPACE FOR ELECTRON FLOW WHOSE LATERAL BOUNDARIES ARE EFFECTIVELY CONSTITUTED BY SAID EMISSIVE SURFACE AND SAID OPERATING SURFACE, SAID SPACE BEING OF UNIFORM CROSS-SECTION IN A SET OF PLANES PERPENDICULAR TO A GIVEN PLANE AND SAID OPERATING SURFACE BEING NON-UNIFORMLY FROM SAID EMISSIVE SURFACE SUCH THAT IN EACH CROSS-SECTION THE PARAMETERS L, D, S AND X DEFINED BELOW SATISFY THE FOLLOWING CONDITIONS: L IS GREATER THAN D; AND OVER THE RANGE OF VALUES OF X FROM ZERO TO 0.9L, S IS FINITE AND INCREASES WITH X SUCH THAT THE FIRST DIFFERENTIAL COEFFICIENT DS/DX IS NEVER NEGATIVE AND IS NEVER GREATER THAN 1.8, THE SECOND DIFFERENTIAL COEFFICIENT D2S/DX2 IS NEVER NEGATIVE, AND THE VALUE OF S IS NOT GREATER THAN 2D WHERE X IS EQUAL TO 0.6L AND IS NOT LESS THAN 1.5D WHEN X IS EQUAL TO 0.9L WHERE, IN THAT CROSS-SECTION, L IS THE LENGTH OF THAT PART OF THE TRACE C OF THE EMISSIVE SURFACE OVER WHICH THE NORMALS TO THE TRACE C INTERSECT THE TRACE F OF THE OPERATING SURFACE; S IS THE DISTANCE BETWEEN THE TRACE C AND THE TRACE F AT ANY POINT A ON SAID PART OF C MEASURED ALONG THE NORMAL TO C AT THAT POINT A; X IS THE DISTANCE OF ANY SUCH POINT A MEASURED ALONG C FROM ONE OF SAID PART OF C AND D IS THE VALUE OF S WHEN X IS ZERO; MEANS FOR ESTABLISHING A MAGNETIC FIELD WITHIN SAID SPACE PERPENDICULAR TO ALL THE PLANES OF SAID SET SO THAT, IN OPERATION, WITH THE FURTHER ELECTRODE HELD AT A POSITIVE POTENTIAL WITH RESPECT TO THE CATHODE SO AS TO ESTABLISH AN ELECTROSTATIC FIELD IN SAID SPACE, ELECTRONS EMITTED FROM THE EMISSIVE SURFACE ARE FORMED INTO A BEAM WHICH TRAVELS GENERALLY ALONG SAID SPACE TOWARDS ITS NARROWER END; MEANS FOR APPLYING AN INPUT SIGNAL TO MODULATE THE ELECTRON BEAM WITHIN SAID SPACE, AND MEANS FOR DERIVING AN OUTPUT SIGNAL BY DEMODULATION OF THE ELECTRON BEAM. 